Sphingosine 1-phosphate (S1P) is a bioactive lipid that circulates at triple digit nanomolar levels in plasma and at single digit micromolar levels in whole blood due to S1P's storage in erythrocytes. Circulating S1P is implicated in tonic control of lymphocyte trafficking, maintenance of endothelial integrity and control of heart rate. Blood S1P is degraded on a time scale of minutes. However, we know neither the mechanism(s) whereby S1P is cleared from circulation nor how it is transported from cells to plasma. The interest in developing therapeutics that block S1P synthesis or action add urgency to developing an understanding of how circulating S1P levels are regulated and the acute and chronic consequences of lowering circulating S1P tone. During the past period of support, we discovered new small molecule S1P receptor agonists, antagonists and synthesis inhibitors and we developed mice genetically altered for S1P signaling and metabolism. Further, we added sophisticated analytical chemistry capabilities that enable quantification of sphingolipid species in small samples. Now we propose to build on this success by using our chemical and molecular biology tools to understand S1P biology more deeply. Specifically, we will: (Aim 1) determine the range of circulating S1P levels, test the hypotheses that the lipid phosphatase LPP1 is responsible for plasma S1P degradation and that Spns2 transports S1P and (Aim 2) use S1P synthesis inhibitors to lower circulating S1P acutely and chronically and determine the effects on lymphocyte trafficking, capillary permeability and heart rate. Our extensive experience in S1P chemical biology, as evidenced by our success in discovering S1P receptor agonist and antagonists with drug-like properties, enables us to address important questions about circulating S1P. The answers will provide both increased fundamental knowledge about S1P biology and practical information to guide the further development of S1P-targeted therapeutics. PUBLIC HEALTH RELEVANCE: Sphingosine 1-phosphate (S1P) is a bioactive lipid that circulates in the bloodstream. The circulating S1P influences the immune system by affecting lymphocyte trafficking, helps to keep small blood vessels from leaking and may tonically depress heart rate. However, we know little about how blood S1P levels are controlled or even the range of S1P concentrations in the blood, but we do know that blood S1P is in a state of rapid flux. Due to the recent approval of a drug that mimics S1P for treating multiple sclerosis, there is a surge of interest in drugs that affect S1P signaling. Among these are investigational drugs that decrease S1P levels by inhibiting its synthesis. Using rodents as surrogates for humans, the research proposed will determine the range of normal S1P levels and explore mechanisms whereby S1P is transported from cells to plasma and how S1P is destroyed in plasma. Further, we will determine the effects of blocking S1P synthesis which rapidly lowers circulating S1P levels on lymphocyte trafficking, vascular leakage and heart rate. By executing this experimental plan, we will learn both about basic S1P biology and predict possible adverse events that accompany therapeutic agents designed to block S1P synthesis.